Carex angustata is a member of the C. stricta complex based on the scabrous, red-brown, bladeless ladder-fibrillose sheaths, the veined perigynia, the hypostomic leaves, and the low chromosome numbers. It is distinguished from sympatric members of the group, C. nudata and C. senta, by the rhizomatous habit, the few-veined perigynia, and the scabrous stems and from the often-sympatric C. aquatilis by the scabrous, veined sheaths and the veined perigynia.